Abstract:

A sonar system for detecting underwater acoustic signals includes a
plurality of hydrophone units capable of converting acoustic impulses to
electrical signals, the hydrophone units being substantially vertically
oriented when deployed in a body of water, and the hydrophone units
occupying at least some of the positions of an M×N horizontal
array. Two-dimensional Chebyshev mathematical weighting is applied to the
electrical signals from the individual hydrophone units such that each
individual signal from each hydrophone unit is assigned a respective
weighting number and a numerical value is assigned to each individual
signal corresponding to the strength of the electrical signal as adjusted
by the respective weighting number.

Claims:

1. A sonar system for detecting underwater acoustic signals, which
comprises:a) a plurality of hydrophone units capable of converting
acoustic impulses to electrical signals, said hydrophone units being
substantially vertically oriented when deployed in a body of water, and
said hydrophone units occupying at least some of the positions of an
N×M horizontal array wherein N is the number of rows and M is the
number of columns in the array;b) means for applying two-dimensional
Chebyshev mathematical weighting to the electrical signals from the
individual hydrophone units such that each individual signal from each
hydrophone unit is assigned a respective weighting number according to
its position in the array and a numerical value is assigned to each
individual signal corresponding to the strength of the electrical signal
as adjusted by the respective weighting number;c) means for deploying
said hydrophone units in the body of water.

2. The sonar system of claim 1 wherein a weighting number wNM for
each individual hydrophone unit is the inner product of the linear
Chebyshev weights in both coordinates according to the formula:
wNM=cheb(N×1)×cheb(1.times.M)

3. The sonar system of claim 1 wherein the N×M array is a 6.times.6
array.

[0006]Undersea noise is well known to be highly dynamic and anisotropic.
Surface-generated noise undergoes multipath propagation in the ocean
waveguide arriving at the receiver at a distribution of elevation angles.
Discrete noise contributions from anthropogenic and biologic sources add
acute directionally in azimuth and elevation. Shallow water bathymetry
also contributes by varying the distribution of the noise field across
azimuth. Such anisotropy in the noise field may be exploited to increase
the detection range of a passive sonar array. However, detection
performance can be severely affected by nearby shipping noise through the
array's beam response sidelobes.

[0007]The detection performance of low-frequency passive sonars, may be
severely affected if deployed under heavy shipping conditions. Merchant
ships, tankers and other anthropogenic undersea acoustic sources are very
loud and they may affect detection performance even at long distances
from the receiver. However, energy from the distant ships (which drops by
about 6 dB/octave in source level with increasing frequency) undergoes
water-column absorption and multiple bottom interactions while the energy
from a nearby quiet signal will be less attenuated due to its shorter
range. Therefore, at higher frequencies, clutter from distant shipping
noise is expected to decrease more rapidly than the received energy level
of the target. In addition, high frequency arrays are much smaller, which
reduces cost and makes them attractive components of unmanned vehicles
and expendable systems.

[0008]There is yet need for array designs and signal processing approaches
to satisfy a number of requirements: a high-frequency passive sonar to
mitigate clutter from shipping noise. The sonar system should be easy to
deploy and the number of array elements should be minimized to reduce
data rate and processor demand. To maximize the detection range of the
system, a new design must feature a vertical aperture to exploit the
ambient noise anisotropy. In addition, the system must be inexpensive for
use as expendable units for multiple uses such as adjuncts to ocean
observatories deployable from air or surface platforms.

SUMMARY OF THE INVENTION

[0009]A sonar system for detecting underwater acoustic signals is provided
herein, the sonar system comprising: (a) a plurality of hydrophone units
capable of converting acoustic impulses to electrical signals, said
hydrophone units being substantially vertically oriented when deployed in
a body of water, and said hydrophone units occupying at least some of the
positions of an N×M horizontal array wherein N is the number of
rows and M is the number of columns in the array; (b) means for applying
a two-dimensional Chebyshev mathematical weighting to the electrical
signals from the individual hydrophone units such that each individual
signal from each hydrophone unit is assigned a respective weighting
number and a numerical value is assigned to each individual signal
corresponding to the strength of the electrical signal as adjusted by the
respective weighting number; and (c) means for deploying said hydrophone
units in the body of water.

[0017]FIG. 5 is a diagram of a system for processing the electrical
signals from the hydrophone units.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018]Referring now to FIG. 1, the hydrophone array deployment system 100
includes a float 102, battery 108, tether line 104, electronics package
130, array support 105, hydrophone array 110, and optionally weight 106,
loaded into a sonobuoy canister 101. The sonobuoy canister 101 can be
size A or smaller.

[0019]Referring also now to FIG. 2, the system 100 is shown deployed in a
body of water 10. Deployment can be accomplished by, for example,
dropping system 100 into the water 10 by airplane, surface vessel, or any
other suitable means. Jettison of the contents of the sonobuoy canister
101 can be accomplished by various means such as the impact of the
sonobuoy into the water. Alternatively, a battery 108 (e.g., seawater
activated battery) can power the ejection of the contents by, for
example, firing a squib. Also, back plate 109 can be jettisoned (by water
impact, firing a squib, etc.) to allow the hydrophone array to descend to
the proper depth. Float 102 remains at the surface of the body of water
and preferably includes a transmitter with antenna 103 and the
appropriate battery-powered electronics for converting the electrical
signals from the hydrophones into radio waves for wireless transmission
to a remote receiver. The transmitter 103 is preferably powered by
battery 108. Tether 104 allows the hydrophone array 110 to descend to a
predetermined depth. Tether 104 can typically range up to 250 feet in
length, for example. Electronics package 130 increases the power of the
electrical signal from the hydrophones for transmission to the
transmitter and also includes a microprocessor for converting analog to
digital signals and performing the calculations described below.
Electronic package 130 can also include a compass to facilitate
geographic orientation of the hydrophone array 110. Weight 106 can
optionally be included to facilitate deployment of the hydrophone array
110. Support system 105 includes two to four rigid connecting rods 105a
to maintain the hydrophone array 110 in a fixed orientation with respect
to the electronics package 130.

[0020]The system 100 is "passive." Active sonobuoy systems emit acoustic
signals into the water and listen for the return echo. Passive systems
merely listen for sounds made by underwater craft, e.g., power-plant,
propellers, door closings or other mechanically generated or human
generated noise.

[0021]Referring now to FIG. 3, the hydrophone array 110 includes a
plurality of hydrophone units 120 arranged parallel to each other in an
array which is embedded in a polymeric encapsulant material 111. The
hydrophones are sized and spaced for specific target frequencies, e.g.,
25 kHz, 20 kHz, 15 kHz. The hydrophones are vertically oriented when
fully deployed. The polymeric encapsulant 111 is preferably an
acoustically transparent polyurethane, i.e., having the same acoustic
impedance as seawater (rho-c material). Polyurethane encapsulants
suitable for use in the invention are commercially available from BF
Goodrich Co. and other suppliers.

[0022]Referring now to FIGS. 4A and 4B, hydrophone units 120 are deployed
in an N×M array wherein N and M are the number of rows and columns,
respectively, of positions for hydrophones in a rectangular grid. The
distance between adjacent hydrophone units within a row or column is the
same. N and M can be the same or different whole numbers and preferably
each range from 6 to 10, more preferably 6 to 8, and most preferably 6 or
7. The more hydrophones in the grid the more accurate determination of
the direction of the received acoustic signals. However, the more space
is needed in the sonobuoy canister to store the array. In a preferred
embodiment shown in FIG. 4B a nominal 6×6 grid is altered by
leaving the corner positions vacant. The reason for this modification is
explained below.

[0023]Various types of acoustic transducers can be used to detect acoustic
waves transmitted through the water. For example, the acoustic transducer
can comprise a tube formed at least in part of a piezo material. Piezo
materials can be piezoelectric, which generate an electrical pulse or
current upon receiving a mechanical impulse such as from an acoustic
vibration, or piezoresistive, which change resistance upon receiving a
mechanical impulse. Piezoelectric material can comprise an active
polarized ceramic material, such as barium titanate or lead zirconate
titanate (PZT). The piezoelectric material can, in another embodiment, be
a piezoelectric polymer material, such as polyvinylidene fluoride (PVDF),
or a piezo-rubber composite material. Piezoresistive materials include,
for example, conductive elastomeric polymeric foams or rubbers which
become more conductive when compressed.

[0024]Typically, hydrophones include a central or core conductor, an outer
conductor, and a layer of piezo material disposed between, and in contact
with, the core conductor and outer conductor in a coaxial configuration.
When subjected to mechanical force the piezo material, such as
polyvinylidene fluoride (PVDF) generates an electrical current which is
carried by the conductors. Hydrophone units typically have a diameter of
from about 1/10 inch to about 1/8 inch. Hydrophone units suitable for use
in the invention are known and commercially available for example from
Argotech Inc. of Fort Lauderdale, Fla.

[0025]The system 100 is deployed, for example, by launch from an airplane.
When the sonobuoy enters the water the contents of the sonobuoy are
ejected from the canister. The float remains on the water surface and the
deployable array 110 drops to a predetermined depth.

[0026]A problem associated with sonar detection using such arrays is the
presence of side lobes where partial constructive interference of
incoming acoustic waves takes place. The side lobes cause confusion of
the signals and it is desirable to suppress the side lobes.

[0027]Shading the conventional beamformer of an array can suppress the
sidelobe levels. There is a vast variety of line-array (one dimensional)
shading functions such as Hanning, Hamming, Blackman, Chebyshev,
Gaussian, Kaiser, Bartlett, Hann, Nuttall, Blackman-Harris, modified
Bartlett-Hanning, Tukey, Bohman, Parsen and, of course, uniform weighting
to name a few. A significant feature of the present invention is the
application of two dimensional (2-D) Chebyshev weighting. Full azimuthal
coverage without ambiguity may be achieved by applying 2-D Chebyshev
weighting to a grid-patterned array of vertical PVDF wires. The advantage
of Chebyshev shading is that it offers the ability to control all
sidelobes to any desired peak level.

[0028]A 2-D gridded array designed for 5 kHz must have an element spacing
of half a wavelength at about 7 kHz to mitigate a backlobe ambiguity. In
addition, the minimum array size is 6×6 hydrophone units. A smaller
array results in the appearance of a backlobe that can't be mitigated
with further reduction of hydrophone unit spacing. Applying 2-D Chebyshev
weighting to this 6×6 array results in heavily shaded corners that
hardly contribute to the array's performance. For example, the corner
hydrophone units are shaded to 4% the value of any of the four center
elements of a 6×6 array Chebyshev shaded to reduce the sidelobes to
40 dB below the level at the maximum response axis. Therefore, the corner
hydrophone units may be omitted from the array to reduce the total number
of hydrophone units to 32 in a "trimmed grid array." Elimination of the
corner units reduces the computational demand upon the microprocessor. At
much lower frequencies, shaded beamforming would result in an
omnidirectional beam pattern with no gain against noise. Instead, this
array may be processes as a gradient sensor to form a cardioid beam
pattern in azimuth to maintain a non-zero array gain.

[0029]The 6×6 trimmed grid array consists of 32 elements (hydrophone
units) collecting time variability of the received pressure field. Fast
Fourier Transform (FFT) of the element-level time series results in the
spectral content of the pressure field p across the selected processing
time,

pn(ω)=FFT(pn(t)), (1)

where n is the element number from 1 to N=32, p is the dynamic pressure,
which is a function of time t and angular frequency ω. A polynomial
fit is applied to obtain a simple expression to estimate the flop count
of each Fast Fourier Transform (FFT). The number of flops can be
estimated using the "flops" command of Matlab (version 5.2).

[0030]Element-based adaptive beamforming is applied to each frequency over
L number of snapshots,

R nn ( ω ) = l = 1 L p nl ( ω )
p nl H ( ω ) ( 2 ) ##EQU00001##

where R is the plane-wave reflection coefficient, where water depth H
indicates complex conjugate, or Hermitian, transpose of the pressure
spectrum. The minimum snapshot duration is approximately given by twice
the group delay across the maximum dimension of the array. Since the 22
kHz grid array is about 0.122 meters in diameter, the minimum required
snapshot duration is 0.16 ms. If the sample rate of this array is 44 kHz,
the minimum snapshot duration correspond to an FFT size of eight samples.
The minimum number of snapshots for an accurate estimation of the noise
covariance matrix is equivalent to three times the number of array
elements for full-rank element-level adaptive beam forming (ABF). Once
the covariance matrix is properly estimated, singular value decomposition
is applied to predict its eigenvalues and eigenvectors

R = n = 1 N λ n v n v n H ( 3 )
##EQU00002##

where λn and νn are the nth eigenvalue and eigenvector,
respectively.

[0031]Since the conventional beam array (CBF) beamwidth of the 2-D
Chebyshev-shaded 6×6 grid array is 33 degrees, the number of CBF
beams is about 11. However, the beamwidth of an adaptive beamformer is
much thinner and the number of beams will definitely increase for
complete coverage. Each ABF beam is steered to,

h=exp(ik(x cos(θ)cos(φ)+y sin(θ)cos(φ)+z sin(φ)))

where (x,y,z) are the coordinates of each array element, k is the
wavenumber, and (θ,φ) are the beam steering directions in
azimuth and elevation, respectively. In the case of the array of vertical
wire elements, the elevation is fixed to φ=0. The minimum variance
distortionless response (MVDR) beamformer weights with a white-noise gain
constraint are given by the elements of,

w = ( R + I ) - 1 h h H ( R +
I ) - 1 h , ( 4 ) ##EQU00003##

where I is the identity matrix, ε is the white noise constraint
(usually set to -3 dB), and

( R + I ) - 1 = n = 1 N ( λ + )
- 1 v n v n H . ( 5 ) ##EQU00004##

[0032]The ABF beam spectra are given by,

bn(ω)=(wpn(ω))2 (6)

where an inverse FFT is applied to generate ABF beam-space time series.

[0033]If the noise field is cluttered with a non-stationary component
caused by a dynamic shipping population, the fastest and nearest ships
relative to the array may cross from one narrow ABF beam to others during
the integration time required to estimate the full-rank element-level
covariance matrix. If the ships are not moving, their energy may be
suppressed with just one eigenvalue per ship. However, a moving ship that
crosses from beam to beam over the integration time will require more
than just one eigenvalue for complete suppression.

[0034]To address this problem, reduced-rank adaptive beamforming was
introduced. It consists of selection of the 1<L<N largest
eigenvalues in Eq. (3) to build a smaller L×L covariance matrix
with the purpose of reducing the required number of snapshots and,
consequently, reducing the integration time to limit the number of ABF
beams ships can cross. The trade-off is that by reducing the size of the
covariance matrix, the number of available eigenvalues to suppress the
ships is also reduced and only the L loudest ships would be suppressed
under the best scenario. Reduced-rank ABF may also be applied to
beam-space time-series. Beam-based reduced-rank ABF requires applying a
conventional beamformer to the array pressure spectra,

b m ( ω ) = n = 1 N h mn ( ω )
p n ( ω ) ( 7 ) ##EQU00005##

[0035]where the steering vector is given by,

hmn(ω)=wn exp(ik(xn
cos(θm)cos(φm)+yn
sin(θm)cos(φm)+zn sin(φm))) (8)

and m represents the beam number from 1 to M. The weights, wn, of the
6×6 trimmed grid array are given by 2-D version of the Chebyshev
weights of a line array with -40 dB sidelobe levels,

[0036]More generally, however, for any size matrix wNM is the inner
product of the linear Chebyshev weights in both coordinates:
wNM=cheb(N×1)×cheb(1×M)

[0037]The reduced-rank beam-space covariance matrix is given by,

R m m ( ω ) = l = 1 L b m l
( ω ) b m l H ( ω ) ( 10 )
##EQU00007##

where 1<L<M is the number of snapshots. Singular value decomposition
follows, as in Eq. (6) except in beam rather than element space, to
obtain the L beam-space eigenvalues and eigenvectors of this covariance
matrix. The beam-space pressure spectra are computed by applying the
eigenvectors to the CBF vectors,

dm(ω)=νmbm(ω), (11)

which is inverse Fourier transformed for the beam-space reduced-rank ABF
time-series solution. These are the main operations for flop-count
measurement of a weighted conventional beamformer and an element or beam
reduced-rank adaptive beamformer.

[0038]It can be seen from the array of weights wn set forth in
formula (9) above the weights of the corner positions (0.04) are
relatively low with respect to the other. Hence, these corner positions
of the 6×6 array need not be occupied by hydrophones. The
elimination of the corner hydrophones reduces the number of required
hydrophones from 36 to 32, with little loss of accuracy, thereby saving
valuable space so as to allow the array to be packed within a size A
sonobuoy canister. Moreover, the shaded beamforming described above with
reduced number of hydrophone units reduces the computational demand on
the microprocessor.

[0039]The final 6×6 grid array configuration is illustrated in FIG.
4B. The on axis sidelobes are reduced to -40 dB. The sidelobes along
elevation from the main lobe can't be suppressed through shading because
the length of the vertical wires is uniform in thickness and sensitivity.
These sidelobes could potentially limit the array's ability to fully
exploit the vertical noise anisotropy. However, they do not degrade the
array's ability to mitigate the nearby ship interference through the
sidelobes.

[0040]Conventional computer control systems can be used to process the
electrical signals from hydrophones 120 using the 2D Chebyshev weighting.
Referring to FIG. 6, electrical signals S from the hydrophone units are
sent to a microprocessor M which applies 2-D Chebyshev weighting C to the
individual hydrophone signals, as described above, the strength of each
signal being adjusted by the corresponding weighting value wn to
produce weighted signals W. Microprocessor M performs the beamforming
computations of formula (11) above to provide accurate directional
information regarding, for example, submarine vessels.

[0041]While the above description contains many specifics, these specifics
should not be construed as limitations of the invention, but merely as
exemplifications of preferred embodiments thereof. For example, while the
invention herein is particularly advantageous for military applications
and has been described in terms of detection of submarines, it can
clearly be employed in any situation wherein acoustic detection is
needed, such as oceanographic or other scientific studies, rescue
operations, and the like. Those skilled in the art will envision many
other embodiments within the scope and spirit of the invention as defined
by the claims appended hereto.